The present invention relates generally to an apparatus for initiating the crystallization of a supercooled material such as those used for phase-change thermal energy storage. Such crystallization causes the supercooled material to change from its high-temperature phase to its low-temperature phase very rapidly, and in doing so, surrender its latent heat of transition.
The principles of thermal energy storage in phase-changer materials is well known. As the materials are heated from an initial phase, such as the solid phase, to a stage, such as the liquid phase, energy is absorbed. In the temperature range at which the material changes from one phase to another, more energy is required to raise the temperature an additional increment than to raise the temperature by the same increment when the material is not changing phase. This additional energy required at the phase change of the material is called the latent heat of transition.
The heat required for the phase change from liquid to gas is called the latent heat of vaporization. The heat required for the phase change from solid to liquid and given up in the reverse phase change from liquid to solid is known as the latent heat of fusion. When a material cools, the energy absorbed at the phase-change point is normally given up. Some materials will cool well below the normal phase change temperature, but still retain the latent heat of transition and remain in the higher temperature phase or state. For example, some materials under some circumstances may be cooled below the temperatures at which they normally change from liquid to solid, yet remain in the liquid state, thus still retain the latent heat of fusion. A material in this condition is said to be undercooled or supercooled. It is possible to create conditions in an undercooled material that will cause it to change very rapidly from the high-temperature phase to the low-temperature phase, thus giving up the energy stored as the latent heat of transition or fusion rapidly. The energy so release may be put to practical use in many ways.
Various types of trigger devices have been utilized in the past to initiate the reaction process necessary to obtain the stored heat when desired. For example, in U.S. Pat. No. 708,549 (Heiliger) is disclosed a trigger device which utilizes the frictional rubbing together of two opposed surfaces to initiate a crystallization reaction which releases heat.
In U.S. Pat. No. 4,860,729 is disclosed an apparatus to nucleate the crystallization of undercooled materials. That patent discloses a trigger device which traps a crystallite of the material between two solid objects and retains it there by pressing the objects together with enough force to create sufficient pressure to keep the crystallite isolated between the two solid objects when it is immersed in the phase-change material and to keep it from melting. Thereafter the crystallite is exposed to the supercooled, phase-change material by releasing the pressure and allowing the supercooled, phase-change material to contact the crystallite. It appears that this device may be lacking in flexibility to various types of applications because it requires the constant isolation of the crystallite from the surrounding solution in order to maintain the integrity of the trigger mechanism.
U.S. Pat. No. 4,077,390 (Stanley) discloses a heat pack enclosing supercoolable aqueous sodium acetate solution together with a metallic activator strip which initiates crystallization by the bending of the activator strip.
Other types of therapeutic heating devices which utilize heat generated from a chemical reaction or change are also disclosed in the following U.S. Patents: U.S. Pat. No. 3,643,665 (Caillouette) ; U.S. Pat. No. 3,951,127 (Watson et al.); U.S. Pat. No. 4,451,383 (Arrhenius); U.S. Pat. No. 4,580,547 (Kapralis)
Accordingly, a need exists for a reusable heat pack device to be used in variety of applications.